What is monosomy?

Monosomy is a genetic defect caused by an incomplete set of chromosomes. Several medical conditions are caused by monosomy, some more serious than others. Along with other genetic defects, monosomy can often be identified during prenatal testing, so such testing is recommended for women at high risk. It is certainly possible for someone to live a normal, healthy life with monosomy, just as other people can be severely disabled by the condition.

Like other chromosomal defects, monosomy is considered a type of aneuploidy. Aneupolydia refers to an abnormality in the chromosomes, leading to birth defects or lack of fetal viability. Most of the time it is caused by errors during meiosis, when chromosomes do not divide properly for various reasons. When sperm and egg cells meet, aneupoloidy results in an incomplete set of chromosomes for the developing fetus. In other cases, the error occurs during the complex cell duplication that creates a viable fetus, meaning the defect is present in some tissues but not others, a phenomenon called mosaicism.

In complete monosomy, one side of a pair of chromosomes is simply missing. An example of this type of monosomy is Turner Syndrome, where the X chromosome is missing a complementary chromosome. A number of developmental disabilities are caused by Turner syndrome, including serious problems like congenital heart disease. However, it is also possible for patients to have a perfectly healthy life with the condition, as long as it is well managed.

Sometimes part of a pairing chromosome is transferred, causing a partial monosomy. In this case, the lack of genetic information can cause a variety of problems, including Le Jeune Syndrome, the result of a partial monosomy on chromosome 5. The syndrome is characterized by a number of symptoms, particularly a malformed larynx that makes that the patient's voice sounds strangely high-pitched. Developmental disabilities may also be present.

Prenatal tests such as an amniocentesis can reveal a monosomy. Genetic counseling is important when doing such tests, as the test results can be very complicated. After such tests, a doctor will usually call the parents to discuss the results. A negative result indicates that no abnormalities were detected, while a positive result suggests that a problem may be present and that parents may need to explore various options to treat it. As with all medical tests, false negatives and positives do occur, so parents should always follow up with additional testing and counseling before making a decision on how to proceed.

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